The present invention relates to a thermal controlling method, a magnetic field generator and an MRI (Magnetic Resonance Imaging) apparatus, and more specifically to a method for controlling the temperature of a permanent magnet-type magnetic field generator, a permanent magnet-type magnetic field generator, and an MRI apparatus provided with such a magnetic field generator.
An MRI apparatus acquires a magnetic resonance signal under a magnetic field generated by a magnetic field generator and reconstructs an image, based on the magnetic resonance signal. As one apparatus of the magnetic field generator, there is known one using permanent magnets. A pair of disc-shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween is used in such a magnetic field generator. As the permanent magnet, a magnet composed of an Nd—Fe—B alloy, i.e., a Neodymium magnet is used.
A magnetic field strength of the magnetic field generator varies depending upon ambient temperatures on the basis of the temperature characteristics of the permanent magnets. Therefore, the magnetic field generator is used in a state in which its temperature is being raised to a temperature higher than room temperature, thereby to avoid the influence of a change in room temperature (refer to, for example, a Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-287950
BH curves of a permanent magnet vary with temperature. As shown in FIG. 8(a), for example, a BH curve given by a linear curve L1 at a temperature T1 results in a linear curve L2 parallel-moved downward at a temperature T2 (>T1). This change is reversible and the BH curve is restored to the linear curve L1 if the temperature is returned to T1.
However, the BH curve at the temperature T2 becomes partly non-linear as indicated by a dashed line L3. When an operating point P of the permanent magnet is placed on such a non-linear region, the BH curve is not returned to the linear curve L1 even though the temperature is restored to T1. This is because the operating BH curve at the temperature T2 results in a linear curve L4 parallel-moved further down as indicated by a broken line L4.
On the other hand, when the operating point P of the permanent magnet is placed on a linear region L21 even where the BH curve at the temperature T2 has a non-linear region L22 as shown in FIG. 8(b), the BH curve is restored to a linear curve L1 if the temperature is returned to T1. That is, the temperature characteristic of the permanent magnet is made reversible when the operating point is placed on the linear region of the BH curve, whereas when the operating point is placed on the non-linear region of the BH curve, the temperature characteristic thereof is rendered irreversible.
The operating point of the permanent magnet is determined depending upon demagnetization. The smaller the demagnetization, the higher the operating point (the higher the magnetic flux density). The larger the demagnetization, the lower the operating point (the lower the magnetic flux density). As the operating point becomes high (demagnetization becomes small), it is easy to fall into the linear region. As the operating point becomes low (demagnetization becomes large), it is easy to fall into the non-linear region.
A non-linear region is small at a magnet large in Hcb/Br and Hcj (thus a linear region is large). A non-linear region is large at a magnet small in Hcb/Br and Hcj (thus a linear region is small). They will be shown in FIGS. 9(a) and 9(b) respectively. In both figures, a graph having a slope corresponds to a BH curve, and a slope-free curve corresponds to a JH curve. Incidentally, Hcb indicates retentivity as to a magnetic flux density B, Br indicates residual magnetism, and Hcj indicates retentivity as to magnetization J. Since Hcj is also large when Hcb/Br is large, Hcb/Br is typified by Hcj below.
The magnet large in Hcj may preferably be used to make the temperature characteristic reversible. However, such a magnet becomes extremely expensive because it contains Dysprosium corresponding to a rare element. On the other hand, the magnet small in Hcj is relatively low in cost because it does not contain Dysprosium.